1. Field of the Invention
The present invention relates to a process for the preparation of terephthalic acid from its di-potassium salt. The process produces high purity terephthalic acid which can be used in the direct preparation of high quality polyester fibers.
2. Description of the Prior Art
It is well known to the art that di-potassium terephthalate is a common starting material in the industrial production of terephthalic acid. In such production schemes, the di-potassium terephthlate is generally obtained by the dismutation of potassium benzoate at elevated temperatures. This process, commonly known as the Henkel process in the chemical art, has been the subject of numerous patents. It is described, for example, in French Patent No. 2,143,401.
The conversion of di-potassium terephthalate derived via the dismutation of potassium benzoate has also been the subject of extensive research and development. The most valuable of these processes involve the reaction of di-potassium terephthalate with benzoic acid. Their advantage lies in their ability to produce, in addition to terephthalic acid, potassium benzoate which can be directly recycled in the production sequence to obtain additional di-potassium terephthalate by dismutation.
It too has been proposed to convert di-potassium terephthalate to terephthalic acid using benzoic acid in processes comprising one or more reaction stages. It is generally recognized that the conversion of di-potassium terephthalate to terephthalic acid can be best accomplished in two stages. During the first stage di-potassium terephthalate is reacted with benzoic acid to yield solid potassium hydrogen terephthalate. During the second stage the potassium hydrogen terephthalate is converted to terephthalic acid using an additional quantity of benzoic acid. Such processes are described, for example, in French Patent Nos. 1,170,781, 1,335,202 and 2,140,168, in British Patent No. 975,113 and in U.S. Pat. No. 2,846,468.
Some processes for the preparation of terephthalic acid from di-potassium terephthalate have been proposed in which all of the benzoic acid is fed into a single reaction zone. Such processes are described in Japanese Patent No. 14,575/66 and U.S. Pat. No. 2,846,468. According to these processes, a nearly saturated aqueous solution of di-potassium terephthalate is reacted with excess benzoic acid. When the reaction is complete, the solid phase, consisting of terephthalic acid, benzoic acid and potassium benzoate, is separated and recovered. Thereafter, the solid mixture is suspended in water and heated. This purification treatment must be repeated several times to obtain terephthalic acid of satisfactory acid number. However, it has been demonstrated that the terephthalic acid prepared by these processes is not suitable for the direct preparation of high quality polyester fibers.
Thus, there remains a need in the art for a method of preparing terephthalic acid in a single reaction zone which results in the production of highly pure terephthalic acid which can be used in the direct preparation of high quality polyester fiber.